Pluto Has A New Moon

…which now makes four and counting. The new discovery comes via an eight-minute exposure (long in this context) of Pluto and its environs with the Hubble Space Telescope. Here is an image that may be a little different from what you have seen published elsewhere:

Pluto and Its Moons

This combines images from two dates (June 28 and July 3, 2011) to show the motion of all four moons around Pluto, and provides a scale bar at the bottom. For comparison, the Earth and its moon are separated by about 240,000 miles (386,000 kilometers).

There is a feature of the image that you may find puzzling. I’m pretty sure I know what’s going on, but I searched in vain for confirmation of this in the information available. Here is my best guess for the origin of the black-background area in the middle. The new moon is quite dim, and Pluto and Charon are relatively much brighter. Dim moons can more easily be seen if there is an obscuring strip, either physical or digital, placed in front of the brighter object. It’s akin to blocking the sun with your hand to see nearby objects more clearly.

I’m guessing that at each of the two dates, a short exposure was made to capture the positions of Pluto and Charon, and that this is what you see in the center of the image. A longer exposure was then made to capture the dim moons Nix, Hydra, and the new as-yet-unnamed moon, here designated P4. The central portion of the second image, with its overbright Pluto and Charon, was digitally replaced with the equivalent portion of that first short-exposure image. The long linear features are artifacts, diffraction spikes that come from overexposing Pluto in the long-exposure image.

How big is the new moon? All we can directly measure at this distance is its brightness, and how big it is depends on how reflective it is. Imagine two objects of equal brightness at this distance, one made of coal and one of ice. The coal moon would have to be much larger to reflect as much light as a moon made of ice. The estimates for the diameter of the new moon range from 13 to 34 kilometers (8 to 21 miles), reflecting the range of estimates of the moon’s reflectivity.

The Hubble is a remarkable scientific instrument. We are seeing an object that has at most a diameter equal to the distance between Lynchburg College’s main campus and the Claytor Nature Center (home of the observatory) at a distance of almost 4 billion miles (6 billion kilometers)!

Why bother with this when we will have a spacecraft on the scene in 2015? (New Horizons, the web site for which is here: http://pluto.jhuapl.edu/.) The encounter with Pluto will be a flyby, meaning that all the scientific observations have to be planned far in advance, with no chance to do on-the-fly adjustments, and without the leisure of sitting in orbit around the planet reacting to changing circumstances. This is especially true when the one-way travel time for radio communications is more than 5 hours. Knowing as much as we can about the system in advance of going there will let us plan the precious time available more efficiently.

Despite its demotion to dwarf planet status, Pluto is still an object of great interest to solar system astronomers. Just as the Dawn mission to the asteroids has turned Vesta from a dimly perceived object to a distinctive place (see the two images below, one taken with the Hubble and one taken by Dawn from a distance of 9500 miles [15,000 km]), New Horizons will show Pluto and its moons in illuminating detail. Mark July 2015 on your calendar for the Pluto encounter!